1. Field of the Invention
This invention pertains to fluid density measurements and more particularly to high frequency acoustical signal velocity measurements of fluid slurries comprising a mixture of liquids, gases and solid particulates, the ratio therebetween being unknown.
2. Description of the Prior Art
Density measurements of fluids employed in oil well drilling, completion and cementing are integral and necessary in the successful and efficient performance of these operations. Accurate observation and monitoring of drilling mud density has generally been a manual, largely "off-line" operation. The most common device for density determination is the simple mud balance scale, wherein drilling rig personnel periodically scoop up a small mud sample, weigh it on a commercially available scale designed for that purpose (which scale correlates weight to density or at least weight change to density change), and record the reading in the drilling log book. Hourly readings are usually also entered into the driller's report. The readings are informative and are the basis for adjusting the constituent parts or otherwise modifying the fluid makeup when the reading varies from acceptable limits.
There have been attempts to automate the above process using both pneumatic and electronic means. For example, one technique that has been employed with a limited degree of success employs differential pressure in a mud reservoir. However, such a technique, although theoretically feasible, is very susceptible to mud caking. Bouyancy techniques in general are also susceptible to inaccuracies resulting from solids settling and to motion distortion. Radioactive measurements have also been employed, but such techniques are both expensive and hazardous. Most techniques also require that they be employed in the first reserve pit after most of the solids and entrained gases are removed. Therefore, the measurements are not realistic of the actual operating fluid conditions and are capable of being made long after the conditions may have changed.
Densitometers using electronic means have been concerned generally with measuring density changes of fluid whose constitutent parts are a combination of liquids or a combination of liquids and entrained gas. It is known that by measuring the acoustic impedance of such a fluid, the density can be determined. The most common type of densitometer utilizing this principle employs a resonant mechanical component, whose resonant point is determined in part by the fluid being measured. This frequency then correlates to acoustic impedance which, in turn, correlates to density. It is known that the resonant frequency decreases with density and that at high fluid densities, which are normally encountered in drilling fluid slurries, the acoustic impedance is relatively constant. Thus, such densitometers are virtually inoperable for making meaningful measurements in the environment of drilling fluids even though acceptable in the environment of lighter, cleaner and more well-defined pipeline fluids.
Ultrasonics have previously been employed in experiments for making density measurement purposes. Again, using fluids comprising liquid and gas mixtures, it has been possible to measure the length of time for a high frequency pulse to pass through a predetermined distance of the fluid. The more dense the fluid, the faster the propagation of the pulse. Hence, it has been possible to correlate travel time to density in such a fluid.
However, the use of ultrasonics in slurry type drilling fluids have not been successful. The major problems appear to be overcoming the dispersion of the transmitted pulse as it encounters the microscopically fine particles of the extremely dense solid materials in the fluid, typically particles of barite. To appreciate the problems associated with developing an automatic, and continuous process for measuring drilling mud density, consider the overall character of drilling mud.
Mud consists generally of a liquid vehicle, usually water, oil or diesel fuel, and contains thickening and weighting ingredients of a ground, fine-flour consistency. Furthermore, the drilled solids of varying size and density circulate in the mud, as do entrained air and gas, as well as varying amounts of salt and fresh water encountered during the drilling process. If permitted to remain still, mud tends to "gel" and suspended solids will, in time, settle out or build up on wetted surfaces. Despite vigorous agitation, mud can typically develop "strata" of varying density in the mud tank system. Drilling mud varies in consistency depending on additives and bottom hole conditions from light, easily flowing mud of table cream consistency to the heavy consistency of grease that barely flows when pumped. Mud thickness also varies with liquid temperature and its chemical make-up. Thick muds are known to be a problem in clogging mechanical apparatus and the high pH of most muds tend to corrode all materials except stainless steel.
It has also been true that anything in contact with drilling fluid normally experiences a build up on the wetted surface that are temporarily exposed to air. This build up has frustrated mechanical designers in attempting to employ bouyancy sensors, differential pressure sensors, continuously-flushed mechanical scales, and other methods of sensing mud density. Rotary joints tend to stick, balance devices tend to gum up and sensitive pressure sensors are battered by the turbulence of mud agitators which have been employed. Therefore, it was recognized by the inventor herein that it would be greatly advantageous to avoid the mechanical difficulties of measuring density using devices with those commonly employed in the prior art. Ultrasonic frequency signals which were not felt to be acceptable for use with mud systems, has been adapted as hereinafter described by the inventor in a novel and unique manner to effectively overcome the difficulties described above.
Therefore, it is a feature of the present invention to provide an improved ultrasonic frequency densitometer for the measurement of the density of drilling fluids, completion fluids and fluids of like consistency, which densitometer is usable under the rugged conditions that exist in the oil field and similar environmental situations.
Another feature of the present invention is to provide an improved method of measuring a minimum number of parameters of a fluid, utilizing a system with no moving mechanical parts, for deriving sufficient information for practical and accurate determination of fluid density.
Yet another feature of the present invention is to provide for the improved utilization of digital processing circuitry for all mathematical computations, which minimizes or eliminates calculation efforts, even under rugged environmental conditions.
Yet another feature of the present invention is to provide for the utilization of ultrasonic wave impulses for the measurement of acoustic velocity of sound waves in a liquid slurry comprising fluid and minute solid particles and the utilization of sound velocity information for the calculation and display of the slurry density.